Modern autorack cars, which is to say autorack cars built since about 1980, have typically had the structure of a flat car underframe covered by a surface deck for supporting automotive vehicles. Since automobiles are, overall, very low density lading as compared to commodities such as coal, sand and gravel aggregate, grain or ores, the flat car underframes tend to be limited by allowable truck center spacing, rather than by the gross weight on rail limit. Currently, autorack cars are typically 89 or 90 ft long as measured over the strikers and as much as 66 ft between truck centers.
The autorack car underframes carry a housing that may tend to resemble a barn. This superstructure is typically referred to as the “rack” of the autorack. In earlier eras the racks were open. However, open racks invite theft and vandalism. Thus modern autorack cars include full side panels (which may have perforations to permit ventilation) and fully closing end doors, with the intention of discouraging thieves and vandals. Most typically the housing structure includes a series of vertical posts spaced along the sides of the car, and one or two additional decks spaced upwardly from the main deck, and upon which respective second and third layers of automotive vehicles may be transported. That is, the rack may be a bi-level rack (i.e., a single elevated deck spaced upwardly above the main deck of the underframe) or a tri-level rack (two upper decks rather than one). These decks are carried on some kind of framework of sideposts spaced along the car, with diagonal bracing or shear web panels between the posts, as may be. The cars tend to be as tall as permitted under AAR Plate ‘F’, namely 20′-2″ maximum height above Top of Rail (TOR). This housing may tend to have gable ends, those ends being open to permit circus loading of the cars, i.e., sequential loading of the automotive vehicles by driving in one end, and out the other on arrival. Although other kinds of doors are known, most typically radial arm doors are mounted at the gable ends and are movable between open and closed positions to govern loading and unloading of the cars. In at least one version of autorack, used for delivering highway tractors to market, there are no additional elevated decks. The racks are typically replaced twice during the economic life of the autorack car underframe. That is, the old set of racks is removed from the underframe and replaced with a new set of racks.
The underframe traditionally provides resistance to vertical bending. Given the great length of span between the truck centers, the center sills of autorack car bodies have tended to be “fish bellied”. That is, the center sill is relatively shallow at the ends of the car at or adjacent to the centerplates over the trucks (i.e., at the truck centers), and rather deeper in the middle between the trucks, perhaps with a downwardly bent, or curved, or deviated, bottom flange. A fish-belly center sill is therefore a fabricated sill in which the bottom profile of the center sill webs is not level, but rather defines the profile of the depth of the beam as a function of longitudinal position. The bottom flange, or bottom cover plate of the center sill is not flat and level, but rather follows the fish-belly profile. The bottom flange may be formed of sections of plate welded to the webs and butt-welded end to end. The sections of the bottom flange may not be co-planar. That is, the fish-belly causes a vertical slope discontinuity. Where there is an abrupt change in properties in the flange, there may also be a corresponding discontinuity in the stress field. In the general case, the predominant loading modes in the bottom flange are (i) longitudinal tension induced by bending, and (ii) longitudinal tension or compression induced by buff and draft loads.
The inside width of section of the center sill is typically 12-⅞ inches. The depth over the centerplates, (i.e., corresponding to the depth of a standard draft sill) is typically 12-16 inches. The depth in the fish-belly is typically about 22 inches, more or less, at the mid-span location between the trucks and along much of the distance along that span between the trucks. A conventional autorack center sill may thus tend to be rather heavy and deep.